The manufacture of semiconductor devices is tending toward greater degrees of automation in the search for higher yield and throughput. One manifestation of this tendency is the use of cassette-to-cassette wafer processing machines. Such a machine accepts an input cassette containing a plurality of wafers, processes those wafers in an automated fashion and returns the wafers to the same or another cassette to be removed from the machine. Many of the process steps used in semiconductor manufacture have been incorporated into such machines. Plasma processes, such as plasma etching and reactive ion etching, are prime examples.
A shortcoming of present automated wafer processing machines is their relative inflexibility to be modified to meet the changing needs of a production line. For instance, a typical cassette-to-cassette plasma etcher having a single plasma reactor cannot be easily modified to add a second plasma reactor. This requires the purchase of an entirely new machine to gain the throughput advantages of a dual head plasma etcher. Improvements in both the hardware of wafer processing machines and in the methods of wafer handling therein are necessary to overcome this inflexibility.
Typical automated wafer processing machines incorporate complex electromechanical and pneumatic systems under the control of one or more microprocessors. In such systems it is necessary that the controllers have knowledge of the present state of each of the moveable parts of the system. This knowledge is particularly necessary when bringing the machine up at some state in the middle of the process. An example of this is the restoration of power following an interruption in the power supply in the midst of operation. Typical prior art systems employ a complex system of sensors distributed throughout the machine to provide the required machine state knowledge. The complexity of this system itself substantially contributes to the failure rates and time to repair of the entire machine.